Method and apparatus for working up a sample of liquid

ABSTRACT

A method and apparatus act on a sample liquid of a pulp and paper industry process water, such as a pulp slurry or white water. The sample is acted upon in an apparatus including sample flow chamber, a filtrate flow chamber, and a filter connecting the sample flow chamber with the filtrate flow chamber. A flow sample liquid, including solid substances such as fibers and filler materials (as well as dissolved and colloidal substances) is introduced through an inlet into the sample flow chamber. A main flow of the sample liquid is discharged through an outlet. A flow of filtrate is passed through a filter from the sample flow chamber into the filter flow chamber, thereby separating a predetermined fraction of solid substance from a minor portion of the introduced flow of sample liquid, in the filter. The minor portion of sample liquid corresponds to less than 1%, typically less than 0.1%, of the total introduced flow of sample liquid. A mat of solid substances is prevented from building up on the inlet side of the filter by inducing in the sample flow chamber turbulence or a high flow velocity of sample liquid adjacent the filter.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention refers to a method and apparatus for working-up asample liquid of a pulp and paper industry process water, such as pulpslurry or white water, in an apparatus including a sample flow chamber,a filtrate flow chamber and a filter connecting said sample flow chamberwith said filtrate flow chamber.

Due to environmental concerns and the rising cost of chemical additives,paper mills are tending to increasingly close the wet end of the papermachine. This leads to accumulation of dissolved and colloidalsubstances (DCS), originating from the wood fibers and, as aconsequence, process disorders. Deteriorated runability and/or inferiorpaper quality are frequently experienced. Various chemicals are added toreduce these problems, but the physio-chemical and chemical interactionsbetween DCS and such compounds are complex and often poorly understood.As a result, an overdosage and/or poor utilization of additives is awell known scenario experienced in paper mills.

Considerable laboratory work has been carried out to characterize thechemical nature and behavior of DCS. Such work has provided new andvaluable insight into some of the interactions between these components.In order to gain a better knowledge of the dynamics of the chemicalinteractions, a continuous monitoring of DCS would be of greatadvantage.

In order to characterize a sample, extraneous material that may hinderor even prevent monitoring and/or analysis equipment from functioning,should be removed. In order to analyze DCS components in e.g. a papermachines white water, varying amounts of fibers and fines present,should be removed.

These fibers (>20 μm) could theoretically be removed from the DCS (<10μm) by size exclusion. Traditional techniques of filtration, however,show the tendency to remove colloidal substances, as well as, fibers,due to an absorption of lipophilic droplets and colloidal particles inthe fiber mat formed on the filter. Successful laboratory methodsutilize a centrifuge. A method of continuous centrifugation has beenproposed, however this requires a large, expensive and inconvenientdecanting centrifuge.

It is an object of the present invention to provide a method and anapparatus, for working-up sample liquid, in which above problems havebeen minimized.

It is a further object of a preferred embodiment of the presentinvention to provide a method and apparatus of the above type whichallows a continuous monitoring of DCS i process water.

It is a still further object of the preferred embodiment of the presentinvention to provide a method and an apparatus for continuous on-linefractionation of solid and colloidal substances in process water.

Toward the fulfillment of the above mentioned and other objects, themethod and the apparatus according to the present invention arecharacterized by what is stated in the appended claims.

A working-up of a sample liquid of a pulp and paper industry processwater can thereby be achieved according to the present invention by

introducing a flow of sample liquid, including solid substances, such asfibers and filler materials, and dissolved and colloidal substances,through an inlet into a sample flow chamber,

discharging a main flow of such introduced sample liquid through anoutlet from the sample flow chamber,

leading from the sample flow chamber through a filter a flow of filtrateinto an adjacent filtrate chamber, thereby separating in the filter froma minor portion of said introduced flow of sample liquid, correspondingto <1%, typically <0.1% of the total introduced flow of sample liquid, apredetermined fraction of solid substances, and

preventing a mat of solid substances from building up on the inlet sideof the filter by inducing in the sample flow chamber turbulence or ahigh flow velocity of sample liquid adjacent the filter.

According to a preferred embodiment of the present invention a flow offiltrate, corresponding to less than or about 0.1% of the total flow ofsample liquid introduced into the sample flow chamber, is forced to flowthrough the filter. Thereby only a small amount of solid substance isseparated in the filter and added into the remaining main flow of sampleliquid being discharged from the sample flow chamber. The added amountof solid substance is that small that it does not noticeably influencethe consistency of the main flow of sample liquid.

The sample flow chamber is according to a most preferred embodiment ofthe invention a sample flow tank, having a filter inserted flush withthe bottom thereof and connected to a filtrate or receiver chamberarranged beneath the bottom.

A mixing device, such as an impeller with rotor blades or other similarelements, is arranged in the sample flow tank, for inducing turbulenceand preventing build up of solid substances on the filter. Rotor bladesare arranged to move relatively close to the filter, e.g. at a distanceh=10-25 mm from the filter. One or several baffles may be placed closeto the inner side wall of the vertical sample flow tank to increaseturbulence.

The sample flow tank can be a relatively high vertical tank, but canalso have the form of an oblong trough, as long as a high velocity flowof sample liquid or turbulence is induced therein to prevent build up ofsolid material on the filter.

A microporous polymer membrane filter, such as a Whatman Cyclopore™Track Etched Membrane made of polycarbonate or polyester or similarother brand polymer membrane, may be inserted in the bottom of thesample flow tank. Such filters usually are very smooth and usually havea thickness of about 7-23 μm, a porosity of about 4-20% and round holeswith a pore size of 0.1-12 μm. Of course other filters, such as wovenfilters and filters with larger pore sizes e.g. up to 70 μm can beutilized.

The flow of sample liquid is forced to flow through the sample flowchamber by a circulating pump arranged upstream or a suction pumpdownstream of the sample flow chamber in the flow of sample liquid.Thereby a high velocity continuous flow of sample liquid is preferablyarranged to flow through the sample flow chamber in order tocontinuously introduce a representative minor flow of sample liquidthrough the filter.

A valve or a suction pump is arranged in the flow of filtrate downstreamof the filter, for controlling the filtrate flow through the filter. Thevelocity of the filtrate through the filter should be much less than 10mm/s, typically about 1 mm/s or even less.

The sample flow chamber is according to another embodiment of thepresent invention a cylindrical through flow housing, having an inlet inone end and an outlet in its other end and further having at least asegment of its cylindrical wall made of filtration medium. The flow ofsample liquid is forced to flow at a high velocity along the filtrationmedium, for preventing build up of solid substances on the inlet side,i.e. the sample flow chamber side, of the filter.

The present invention provides a new sample work-up method and apparatusfor continuous on-line fractionation of pulp and paper process waters.Pulp fibers and large fines are selectively removed from the requiredanalytical sample by utilizing a filtration medium as a size exclusionbarrier. A high flow rate of process water is passed across one side ofthe filter medium with only a relatively small volume of sample passingthrough the filtration medium. Thus a filter cake and subsequentclogging of the membrane can be prevented as the process waterconsistency is not significantly reduced by the filtration, due to thesmall filtrate flow and a fast process water turnover and as fibers arecontinuously stripped from the filter surface due to turbulence, shearand eddy effects, induced by mechanical turbulence inducing means and/orhigh flow rates.

In the traditional sense, filtration is the separation of a fluid solidsmixture involving the passage of most of the fluid through a porousbarrier which retains most of the solid particulate contained in themixture. Usually the filtrate is transferred through the filter eitherby pressure applied upstream to the filter medium or by vacuum appliedto the filtrate.

The new filtration technique is, however, based on the premises thatonly the quality of filtrate is critical and that the yield of filtrateis of minor importance. We have found, when working-up samples of pulpslurry, that it is possible to eliminate the buildup of a fiber mat (andsubsequent absorption of lipophilic droplets) by increasing theturbulence or the flowrate of pulp slurry adjacent the filter and bysubstantially decreasing the flow of filtrate compared to traditionalfiltration.

Following mechanisms are believed to have an positive impact on the newfiltration technique, when working-up a good analytical sample fromprocess water, such as pulp slurry:

a) Flow of filtrate through filter:

We have found that the flow velocity of filtrate through the filtermedium has a great impact on the flow conditions through the filtermedium. Too high flow velocity tends to build up a filter cake in thefilter. The flow velocity through e.g. a filter with 10 μm pores, suchas Whatman Cyclopore™ membrane, should not exceed 10 mm/s, but shouldtypically be about 1 mm/s, preferably even less 10-15 mm/min. Highpressure difference easily causes cake formation on filter surface. Thepressure difference over the filter should be less than 0.1 bar,preferably negligible. An agitator, such as a magnetic agitator, may beprovided on the outlet side of the filter, if a cake of fines or similarsubstances tends to build up on the outlet side. Also the filtrateflowrate should be as small as possible, e.g. 10-60 ml/min, depending onthe effective filter area, may be enough, in order not to cause hold upand integration of filtrate, where momentary variations in filtrateconditions are diluted in large filtrate volumes.

b) Consistency of sample liquid (process water):

We have found that the factor of process water, e.g. pulp slurry,consistency is important since the quantity of solids present in thewater close to the filter has an impact on whether a cake will be formedor not. Local variations in process water consistency over the filtersurface may occur resulting in regions of elevated solids content.However, if the process water is uniformly mixed and the quantity offiltrate extracted has negligible influence upon the process waterconsistency, then such effects are also negligible. Therefore highprocess water turnover is suggested.

c) Flow profile of sample liquid (process water):

Turbulence, eddy currents and shear forces are invoked in order to stripfibers and solids away from the filter surface. It is essential to havesuch a sample liquid flow profile that material is continuously beingremoved from the filter surface by forces within the fluid itself. Theshear forces and eddy forces acting close to the filter surface areessential for preventing fibers from either clogging or passinglongitudinally through the pores of the filter. The fibers that brushpast the filter surface assist additionally by "wiping" the filtersurface of fines and other debris that might collect.

A high flowrate of pulp slurry through the sample liquid chamber alsoprovides a continuous relevant sample of liquid in front of the filter.

d) Effect of the filter itself:

Adjacent the filter surface, on the inlet side of the filter, twodifferent forces tend to pull fibers in different directions. Shearforces tend to strip fibers from the filter surface, whereas flow offiltrate tends to pull fibers into the pores of the filter. The filtershould therefore preferably be very smooth to prevent fibers fromgetting caught by the filter surface and be very thin (e.g. <23 μm,preferably even less) to prevent fibers from being permanently stuck inthe filter pores/capillaries. If needed (e.g. for ultra thin filtershaving a large filter area) the filter can be supported from thefiltrate side.

Low flowrates across filters utilized in the system according to thepresent invention and accordingly low almost negligible pressure dropsover these filters allow very thin filters to be used. Alreadyrelatively low pressure drops could mechanically damage ultra thinfilters, e.g. of a few micrometer thickness.

Depending on pore size certain fractions of fibers, such as fines, andcolloidal substances flow easily through thin filters. Larger fibers maymomentarily get trapped with their one end in the pores, the other endstill protruding out of the filter surface (the filter being very thin).Such protruding fibers are easily stripped off the filter surface byturbulent flow conditions on the inlet side of the filter. Preferablythe distance between adjacent pores in a thin filter is (if practicallypossible) large enough not to allow one fiber from being simultaneouslystuck at its both ends in adjacent pores. A thin filter, if momentarilyclogged may easily be regenerated by introducing a high velocity flow orhighly turbulent flow of pulp slurry over the filter surface, while theflow of filtrate is temporarily halted.

The filter pore size may be selected according to need. A filter havinga pore size of about 0.5 μm may be used to separate fines and colloidalsubstances, whereas a filter having a pore size of about 70 μm may beused if a filtrate including larger fines is needed.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of this invention will become apparent inthe following detailed description of the preferred embodiments withreference to the accompanying drawings, in which

FIGS. 1a and 1b show schematical drawings of apparatuses according tothe present invention;

FIG. 2 shows schematically and in more detail a sample tank, the filterand filtrate chamber separated from the sample tank;

FIG. 3 shows schematically a partly elevational view of another samplechamber according to the present invention;

FIGS. 4a and 4b show experimental apparatuses used in experiments 1-4and

FIGS. 5 to 7 show turbidity charts for experiments 1-4.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1a shows a sample tank 10, having an inlet 12 and an outlet 14 forsample liquid. A plane microporous filter 16 is inserted flush with thebottom 18 of the sample tank, for providing a flow connection with afiltrate chamber 20 beneath the sample tank.

An impeller 22 is positioned in the sample tank 10 at a short distance(h) from the filter, for providing turbulence in the sample tank.

A flow of sample liquid, such as pulp slurry or white water, is pumpedwith pressurizing pump 24, upstream of the sample tank, through line 26and inlet 12 into the sample tank and out of the tank through outlet 14and line 28. Filtrate is discharged from the filtrate chamber 20 throughline 30. This filter system operates by over pressure and valve 32 inline 30 controls the flow of filtrate.

FIG. 1b shows another primary scheme of operation of a sample tankconnected to a filter chamber. A pump 24' is placed downstream of thesample tank. A vacuum pump 25, such as a peristaltic pump, is placeddownstream of the filter to ensure that the filtrate is under approx.the same pressure (vacuum) as the sample flow.

FIG. 2 shows in a side view a sample tank 10, having arranged axially inits center an impeller 22, with 4 rotor blades 34 close to the bottom ofthe tank. The filter 16 and the filtrate chamber 20 and a base plate 36,have been taken apart from the sample tank 10. The filter 16 is aWhatman Cyclopore™ membrane filter. The tank can be made of stainlesssteel or any other suitable material.

FIG. 3 shows a cylindrical flow through sample chamber 10, having aninlet 12 at its first end and an outlet 14 at its other end. Cylindricalfilter segments 16', e.g. of woven polyester cloth, are inserted in theside walls 38 of the cylindrical chamber. Means not shown are providedto introduce a flow 40 of sample liquid, such as process water, into thesample chamber. A main flow 42 of the sample liquid is dischargedthrough the outlet and a minor flow 44 is directed to flow through thefilter.

Experiments have been performed in order to study effects of differentimpeller speeds, filtrate flowrates, additions of filler materials andeffect of baffle on filtration conditions of a test apparatus asschematically shown in FIG. 4a and FIG. 4b. A bulk sample of 1,5 wt %TMP (thermomechanical pulp) slurry was introduced through inlet 12 intoa sample tank 10 (φ 280 mm and volume 20 1), having an impeller 22 withrotor blades 34. A filtrate was withdrawn through the filter 16 (φ 142mm, thickness 10 μm, pore size 10 μm and porosity 8%) and recirculatedthrough line 30' into the tank 10. Turbidity of the filtrate wasmeasured with on-line measuring apparatus 36, to give a value onturbulence, flocculation and sedimentation tendency of fibers and finesin the tank.

EXPERIMENT 1

Effects of different impeller speeds were studied in an apparatus asshown in FIG. 4a. A bulk sample of 1,5 wt % TMP (thermomechanical pulp)slurry without fillers was introduced into the sample tank 10. The rotorblades 34 of the impeller was placed at a distance h=25 mm above thefilter membrane 16. A sample flowrate of approx. 10 ml/min was withdrawnfrom the tank 10 through the filter 16 and circulated back through aline 30' to the bulk slurry in the tank 10.

First the effect of a variation of impeller speed in the range of 200 to2000 rpm was studied. Thereafter a more detailed study was undertakenwith impeller speeds in the range of 1000 to 400 rpm. The impeller speedwas altered only when the turbidity reading was deemed stable. No otherparameters were varied during this experiment.

Impeller speeds >1000 rpm resulted in considerable vortexes formingwithin the tank and membrane filter damage. Impeller speeds less than300 rpm resulted in cake formation on the filter.

The most stable range of operation appeared to exist between 400 and1000 rpm as no filter damage or cake formation occurred during theseexperiments. It appears that for the given conditions, impeller speed islinearly proportional to the change in turbidity. A turbidity chart forvarying impeller speeds in the range of 1000-400 rpm is shown in FIG. 5.

EXPERIMENT 2

Effects of different filtrate flowrates was studied in an apparatusshown in FIG. 4a. A bulk sample of 1,5 wt % TMP slurry without fillerswas introduced into the sample tank 10. The rotor 34 was placed approx.25 mm centered above the filter membrane 16 and set to 600 rpm.

The filtrate flowrate was set to approx. 600 ml/min, 60 ml/min, 30ml/min or 10 ml/min. After a few minutes of operation at 600 ml/min athick filter cake formed on the filter 16. The experiment was continuedfor at the lower flowrates (60-10 ml/min). The obtained turbidity chartsare shown in FIG.6. Extremely low flowrates (10 ml/min) resulted infines flocculation and sedimentation in the receiver chamber below themembrane filter. A stirrer may be disposed in the receiver chamber toavoid this. 60 ml/min appeared to be a good choice of filtrate flowratefor this experiment.

EXPERIMENT 3

Effects of addition of filler materials in the bulk sample was studiedin apparatuses shown in FIG. 4a and 4b. The apparatus shown in FIG. 4bdiffers from the apparatus in FIG. 4a in that a baffle 42 is placedvertically on the side wall of the sample tank 10 in order to increaseturbulence therein.

A bulk sample of 1,5 wt % TMP slurry with 0,5 wt % clay filler materialwas introduced into the tank 10. The first part of the experiment wasperformed in an apparatus shown in FIG. 4a with the rotor 34 placedapprox. 25 mm centered above the filter membrane 16. In the second partof the experiment the apparatus was modified by including a singlewooden baffle 42 into the tank, as shown in FIG. 4b. The baffle waspositioned approx. 10 mm from the base of the tank 10. Both filtrateflowrate and impeller speed were varied in the experiments.

The experiment showed that a TMP slurry including filler material led toa filter blockage after ca. 15 minutes of operation when using anapparatus without a baffle, as shown in first (left) part of turbiditychart in FIG. 7. The addition of a baffle, as shown in FIG. 4a, resultedin the disruption of the vortex formed in the tank and a considerablyincreased turbulence within the tank.

Flowrates greater than 100 ml/min and impeller speeds less than 200 rpmresulted also in cake formation and subsequent blockage, when using theapparatus shown in FIG. 4b. However, with impeller speeds keptsufficiently high (>500 rpm) and flowrates kept low (<60 ml/min) theapparatus operated, without filter fouling, for extended periods (>1000min) and a consistent filtrate was obtained, as can be seen from thesecond (right) part of turbidity chart shown in FIG. 7.

It can be concluded from the experiment that the increased turbulence ofthe bulk sample by incorporation of a baffle prevented the formation ofa filter cake, i.e. turbulence prevented the sedimentation of theheavier filler materials.

The membrane filter material used is extremely resilient to harshtreatment. Even after successive periods (several hours each) of severecake formation with significant under pressure in the receiving line,the filter could be regenerated. The method of regeneration wasstraightforward and required no backflush as would be needed withtraditional filter matrices.

The procedure was simply to stop the filtrate flow, remove the baffleand to use high impeller speeds (around 1500 rpm) for approx. 1 minuteand thus wipe the filter surface clear of debris. Following this, theimpeller was set at the original value (600 rpm), the baffle wasreinstalled and the filtrate pump was restarted (60 ml/min).

The experiment shows that the turbulence of the bulk sample (above thefilter) in the tank is essential to prevent cake formation andsubsequent filter blockage. A filtrate flowrate of approx. 60 ml/mingave consistent results. Higher flowrates (>100 ml/min) resulted in cakeformation and subsequent filter blockage.

EXPERIMENT 4

Effects of further addition of filler materials and the increase ofturbulence by altering the location of the propeller was studied. A bulksample of 1,5 wt % TMP slurry with 1,0 wt % filler material wasintroduced into the tank shown in FIG. 4b (filtrate flowrate; 60 ml/min,600 rpm rotor speed).

After approx. 2 hours of operation a cake gradually formed on the filterleading to complete blockage. The turbulence of the bulk slurry wasincreased by lowering the rotor 34 of the impeller from about 25 mm to10 mm above the filter surface. This increased the turbulence of thebulk slurry and restored the system. At a flowrate of 60 ml/min, astable turbidity measurement was obtained for an extended period oftime.

As an over all c conclusion can be noticed that:

A too high flowrate (>100 ml/min) resulted in filter fouling.

Extremely low filtrate flowrates (<10 ml/min) resulted in excessivefines accumulation within the system.

A suggested ideal operating flowrate is 60 ml/min. This is of coursedependent upon the size and porosity of the filtration medium. In termsof velocity through the membrane pores, this relates to approx. 1 mm/sfor the filter used in these experiments.

High turbulence is required in order to keep the bulk pulp and fillerslurry suspended and to prevent cake formation and subsequent filterfouling.

The type of filter material is extremely critical for this experiment towork. The ability of the used filter to avoid damage through poreblockage is a key feature.

In above experiments a special very smooth polycarbonate filter membranehaving a thickness of 10 μm and pore size of 10 μm has been used. It isobvious that other types of filters, such as woven filters, can be usedin an apparatus according to the present invention. Larger pore sizes,e.g. up to 70 μm, can be used as long as, consistent filtrate isachieved. Larger pore size filters can e.g. be used in connection withretention measurements.

The present invention provides a fast and reliable method and apparatusfor separating fibers and fines from a sample flow of pulp slurry beforeanalyzing colloidal and dissolved substances. The invention alsoprovides means for testing the impact of different chemicals on aliquid, such as a pulp slurry. The invention can e.g. be used to measurefines passing the filter when using different retention agents, in orderto compare the effect of different agents.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to theenclosed embodiments, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

We claim:
 1. A method of obtaining a filtered sample for thedetermination of at least one of dissolved and colloidal substances,using a sample flow chamber, a filtrate flow chamber, and a filterconnecting the sample flow chamber with the filtrate flow chamber,comprising:(a) introducing a flow of sample liquid, including solidsubstances and at least one of dissolved and colloidal substances, intothe sample flow chamber; (b) discharging a primary flow of sample liquidfrom the sample flow chamber; (c) separating a minor flow of sampleliquid from the sample flow chamber corresponding to less than 1% of theflow of sample liquid from (a), and passing the minor flow through thefilter into the filtrate chamber to produce a solid substance removedfrom the flow of sample liquid to provide a filtered sample in thefiltrate chamber which is suitable for use for the determination of atleast one of dissolved and colloidal substances therein; and (d)inducing turbulence or a high flow velocity of sample liquid in thesample flow chamber adjacent the chamber sufficient to prevent a mat ofsolid substance from building up on the filter.
 2. A method as recitedin claim 1 wherein the sample liquid comprises a pulp and paper industryprocess liquid; and wherein (c) is practiced to provide less than 0.1%of the flow of sample liquid from (a) as the minor flow passed throughthe filter.
 3. A method as recited in claim 2 further comprising usingthe filtered sample from (c) to determine the content of at least one ofdissolved and colloidal substances therein.
 4. A method as recited inclaim 3 wherein (c) is further practiced by passing the liquid throughthe filter at a velocity of about 1 mm/sec.
 5. A method as recited inclaim 1 wherein (c) is practiced so as to not noticeably influence theconsistency of the flow of sample liquid in (a).
 6. A method as recitedin claim 1 wherein (d) is practiced using an impeller rotating adjacentthe filter.
 7. A method as recited in claim 1 wherein the sample flowchamber comprises a substantially cylindrical flow chamber having aninlet at a first end, and an outlet at a second end opposite the firstend, and wherein the filter is in a side wall of the cylindrical flowchamber so that liquid passing through the filter moves substantiallyperpendicularly to the direction of flow of liquid from the inlet to theoutlet; and wherein (d) is practiced solely by passing the sample liquidat high velocity along the filter.
 8. A method as recited in claim 1wherein (c) is practiced by passing the liquid through a polymermembrane filter having a thickness of about 7-23 microns, a porosity ofabout 4-20%, and a pore size of between 0.1-12 microns.
 9. A method asrecited in claim 1 wherein (c) is further practiced by passing theliquid through the filter at a velocity of less than 10 mm/sec.
 10. Amethod as recited in claim 1 wherein the sample liquid comprises a pulpand paper industry process liquid; further comprising using the filteredsample from (c) to determine the content of at least one of dissolvedand colloidal substances therein.
 11. A method as recited in claim 10wherein (c) is further practiced by passing the liquid through thefilter at a velocity of less than 10 mm/sec and by controlling the flowof liquid through the filter by controlling a valve on the opposite sideof the filter from the filtrate flow chamber.
 12. A method as recited inclaim 1 wherein (a)-(c) are practiced at least in part by applyingpressure to the liquid being sampled from upstream of the sample flowchamber, or by applying vacuum thereto downstream of the sample flowchamber.
 13. A method as recited in claim 12 wherein (c) is furtherpracticed by controlling the flow of liquid through the filter bycontrolling a valve on the opposite side of the filter from the filtrateflow chamber.
 14. Apparatus for obtaining a filtered sample for thedetermination of at least one of dissolved and colloidal substances,comprising:a sample flow chamber having a side wall and a bottom, andconstructed so that a continuous flow of sample liquid may be providedtherethrough; a filtrate flow chamber; a filter connecting said sampleflow chamber with said filtrate flow chamber, said filter disposed insaid side wall or bottom of said sample flow chamber; and means forcausing a flow of the sample liquid from said sample flow chamberthrough said filter corresponding to less than 1% of the flow of sampleliquid in said sample flow chamber, to produce a solid substance removedfrom the flow of sample liquid to provide a filtered sample in saidfiltrate chamber which is suitable for use for the determination of atleast one of dissolved and colloidal substances therein.
 15. Apparatusas recited in claim 14 wherein said means for causing a flow of thesample liquid through the filter corresponding to less than 1% of theflow of sample liquid in the sample flow chamber comprising a conduitleading away from the filtrate flow chamber, and a valve in the conduit.16. Apparatus as recited in claim 14 wherein said means for causing aflow of the sample liquid through the filter corresponding to less than1% of the flow of sample liquid in the sample flow chamber comprising aconduit leading away from the filtrate flow chamber, and a vacuum pumpconnected to said conduit.
 17. Apparatus as recited in claim 14 whereinsaid means for causing a flow of the sample liquid through the filtercauses a flow of the sample liquid through the filter corresponding toless than 0.1% of the flow of sample liquid in the sample flow chamber.18. Apparatus as recited in claim 14 further comprising means forpreventing buildup of solid substances on said filter.
 19. Apparatus asrecited in claim 18 wherein said means for preventing buildup of solidsubstances on said filter comprises a rotating impeller adjacent saidfilter.
 20. Apparatus as recited in claim 18 wherein said sample flowchamber comprises a substantially cylindrical flow chamber having aninlet at a first end, and an outlet at a second end opposite the firstend; and wherein said filter is in said side wall of said cylindricalflow chamber so that liquid passing through the filter movessubstantially perpendicularly to the direction of flow of liquid fromthe inlet to the outlet; and wherein said means for preventing buildupsolely comprises positioning said filter adjacent the flow of liquidfrom said inlet to said outlet so that the sample liquid at highvelocity along the filter.
 21. Apparatus as recited in claim 14 whereinsaid filter comprises a nonwoven polymer membrane filter having athickness of about 7-23 microns, a porosity of about 4-20%, and a poresize of 0.1-12 microns.
 22. Apparatus as recited in claim 14 whereinsaid sample flow chamber, filter, and filtrate flow chamber comprise afirst sample flow chamber, filter, and filtrate flow chamber, andfurther comprising at least a second sample flow chamber, second filter,and second filtrate flow chamber, disposed in parallel with said firstsample flow chamber, filter, and filtrate flow chamber, said at least asecond filter having a different pore size than said first filter. 23.Apparatus as recited in claim 14 further comprising a first suction pumpfor pulling the flow of sample liquid through said sample flow chamber,and a second suction pump for pulling filtrate through said filter.